The objectives of the current proposal are to characterize the in vitro culture system by which specifically sensitized lymphocytes are generated by co-cultivation with murine mammary tumor cells, to utilize it to ascertain the basic mechanisms underlying the in vivo immune responses to mammary tumors, and to determine optimal conditions for obtaining successful therapeutic effects when lymphocytes sensitized in vitro are transferred into tumor bearing mice. The cytotoxicity exhibited by the in vitro sensitized lymphocytes appeared to be directed against mammary tumor virus associated antigens. When these lymphocytes were transferred into syngeneic mice, they were capable of preventing rapid growth of the corresponding mammary tumors. When these cells were labelled with 51Cr and injected into syngeneic mice, a major portion of activity was detected in the liver. In the coming year, studies will be performed to further investigate the immunotherapeutic effect of the in vitro generated effector cells.